1. Field of the Invention
The present invention is concerned with an improved, composited, short span bridge construction, and method of fabricating the same. More particularly, it is concerned with such a structure and method wherein use is made of structural metallic steel plates composited to concrete such that, after the bridge is erected, the concrete absorbs load-induced compressive forces, and the steel absorbs primary load-induced tensile forces in all planar directions; in this fashion, maximum advantage is taken of the strength characteristics of the materials forming the bridge superstructure without the necessity of embedding reinforcing steel with the concrete decking. In addition, with use of standard mill steel plate, and shop casting techniques, the cost of completed bridges in accordance with the invention is significantly lower than that of conventional designs.
2. Description of the Prior Art
The construction of modern day bridges, such as highway and railroad bridges, is subject to a number of constraints, principally arising from the necessity of adequately distributing and safely absorbing concentrated moving loads imposed thereon without excessive deflection. In the case of highway bridges, the recognition of this problem has led to promulgation of a plethora of rather stringent regulations. For example, one commonly applied code specifies that the completed bridge must be able to sustain, over each possible ten foot travelway within its width, a uniform load of 640 lbs. per lineal foot, and 32,000 lbs. of moving and concentrated load. Additional provisions of the code deal with shear concentrations, and the effects of impact. The bridge must also exhibit sufficient stiffness to strictly limit deflections and oscillations, usually limited to a deflection-to-span ratio of 1 to 800 or less under full loading with impact. A large number of other provisions also exist for ensuring structural stability due to wind, ice, braking impacts and centrifugal effects.
In addition to the foregoing, a bridge must have a relatively long useful life, require only a minimum of maintenance, and have the ability to withstand climatic freeze/thaw cycles and the effects of deicing compounds used for maintenance of the trafficway.
Thus, although it is entirely possible to demonstrate that a typical bar joist commonly used in building construction to support an office floor or roof can serve as a bridgeway for pedestrians or light vehicular traffic, such a construction is in no way related to the service loading or structural requirements of a modern-day highway bridge.
To serve highway traffic needs, a number of bridge structures have evolved as standards. Although a large number of individual types of bridges exist, they can generally be put into a number of classes. One such class of prior bridge is a concrete bridge which can be either poured in place over removable forming or precast. Poured in place bridges generally require significant steel reinforcement, usually in the form of bars embedded in the concrete, and are typically limited to smaller spans. This is because as span length increases with this type of bridge, dead loads likewise increase dramatically. Precast concrete bridges generally consist of three basic forms, the precast tee, precast I girder and segmented box bridges.
Another broad classification of prior bridges is that of steel bridges. This classification includes the I beam bridge, typically used for spans of up to 70 feet. This type of bridge employs a multiplicity of cross braced I beams which are either brought into composite with the concrete bridge deck to improve performance in deflection, or in older bridges act as simple supports for the upper deck. Another type of steel bridge is the plate girder bridge, generally used for spans of from 50 through 120 feet. Plategirders are used in this construction in lieu of rolled beams to achieve the same results but with a deeper section formed by welding the girder web plate to flanges of required section. The orthotropic steel bridge is yet another steel bridge but is typically quite expensive because of fabrication and materials costs. This bridge achieves the requirements of structure through use of a double plated steel structure that distributes loads two ways across its steel decking. The steel truss bridge is another type of prior construction, but is seldom used today because of its expense, and depth-to-span ratio limit of about 1 to 10. A final type of steel bridge is the suspension bridge, which finds application only in extremely long spans.
As the above discussion demonstrates, a number of ways have been proposed in the past to resolve the structural problems inherent in highway bridge construction. The one constant factor present in these prior proposals however, is that the resultant bridge, in total cost, is very expensive. This expense generally arises because of a need for extensive, skilled on-site labor, or large quantities of expensive, especially designed and ordered steel structural members. Indeed, in a number of the bridge types discussed above, both of these problems are present.